* Add a rollup function called cache_unlink() to handle namecache
effects when unlinking a file.

* Change namecache semantics a bit. When a namecache entry is unlinked
we allow it to be left in the topology as long as the vnode survives,
but marked NCF_DESTROYED so it does not conflict with any new entries
that might be named the same, and does not get returned in lookup results.

* This will solve the issue when renaming a file over an empty directory
(destroying the directory) when one or more processes are chdir'd into
that directory. This would cause the process nchdir stuff to get out
of sync with the retained directory vnode because the same namecache
entry would get reused.